The effects of Fe on the oxygen storage and release properties of model Pd–Rh/CeO 2–Al 2O 3 three-way catalyst

Abstract

This work attempts to gain fundamental information about the role of Fe in the chemical deactivation of a three-way catalyst (TWC), with Fe originating from materials used in the construction of automotive engines and exhaust pipe systems. Topics of particular interest are Fe's role as a chemical poison of the oxygen storage and release properties of a model TWC (1 wt% Pd–Rh/20 wt% CeO 2–Al 2O 3), its effects on the redox properties of Pd and Rh, and its effects on NO and CO chemisorption; these topics are investigated for the first time here. Oxygen storage capacity (OSC) measurements in the 500–850 °C range indicated that OSC increased after the addition of 0.1–0.3 wt% Fe in the model catalyst. It can be concluded that iron acted as an oxygen storage component under oxidizing conditions through the process Fe → FeO → Fe 3O 4 (FeO⋅Fe 2O 3) → Fe 2O 3, and not as a chemical poison of the oxygen storage and release properties of the solid. However, when the amount of deposited iron increased to 0.4 wt%, the OSC of the catalyst deteriorated but still remained slightly greater than that measured over the catalyst uncontaminated with Fe catalyst. H 2 temperature-programmed desorption and reduction and X-ray photoelectron spectroscopy (XPS) studies performed on the oxidized and reduced Fe-contaminated samples indicated that iron (0.1–0.3 wt%) was deposited on both noble metals (Pd and Rh) and support, causing the development of electronic interactions that influenced the oxidation states of Pd and Rh and enhanced the oxygen chemisorption properties of ceria. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies proved that iron deposited to a level of 0.2 wt% significantly favors the formation of a superoxide ( O 2 − ) species on CeO 2. The latter species was found to desorb completely at temperatures above 250 °C. However, at the 0.4 wt% level, O 2 − formation was minor. In situ DRIFTS chemisorption studies of CO and NO performed at 25 °C also provided important information about the electronic interactions between Pd and Fe in the model Pd–Rh–Fe/20 wt% CeO 2–Al 2O 3 TWC, confirming the XPS results.